The present invention relates to a tappet for a valve mechanism for an internal combustion engine, for example, an engine for an automobile, and more particularly to a hydraulic tappet made of ceramic and capable of improving the efficiency of the engine for an automobile.
Hitherto, an internal combustion engine, such as an engine for an automobile, has been arranged in such a way that a tappet or a rocker arm is disposed at an end of a valve shaft to transmit, to a valve mechanism, the kinetic force of the cam. Further, thermal expansion or thermal shrinkage of a cylinder head or a valve occurring at the time of the operation and incomplete seating of the valve occurring due to wear of the valve or a valve seat are prevented and the kinetic characteristics of the valve system are improved by disposing a hydraulic or mechanical adjustment mechanism in a portion in which the tappet or the rocker arm is in contact with the end of the valve shaft.
FIG. 5 illustrates an example of a conventional tappet of a hydraulic adjustment type. If a cylinder head (omitted from illustration) or a valve 18 encounters thermal expansion or thermal shrinkage, the hydraulic pressure of an oil reservoir disposed in a metal tappet 10 is used to adjust the position of the spring portion for pressing the valve of the tappet so that the transmission characteristics from a cam 17 are compensated.
However, a conventional tappet of the foregoing hydraulic adjustment type involves generation of excessively large frictional force when a cam sliding portion 12 slides on the cam 17 of the metal tappet 10 as shown in FIG. 5. As a result, there arises a problem in that the crank shaft must bear an excessively heavy load, and therefore the efficiency of the engine deteriorates.
Moreover, the employed mechanism, in which oil is enclosed in the tappet, raises the ratio at which the foregoing oil reservoir occupies the inside portion of the tappet, and the weight of the tappet is increased excessively, causing a problem to arise in that the transmission loss occurring due to the vertical motion of the valve 18 becomes critical.
In a recent circumstance in which a study has been made in such a manner that the valve is made of ceramic material to reduce the weight to make smooth the movement of the valve so as to improve the efficiency of the engine, use of the conventional metal tappet encounters a problem that the weight of the valve mechanism cannot satisfactorily be reduced, and accordingly the desired effect of using the ceramic valve cannot be obtained.